A developer device (developer unit) capable of accommodating a developer therein is disposed in an electrophotographic type image forming apparatus such as a laser printer or a digital multi function device. Then, the developer accommodated in the developer device is supplied to a developing roller through a supply roller and is further supplied from the developing roller to an image carrier on which an electrostatic latent image is formed, and thereafter is transferred to a recording sheet.
Since a developer is generally a consumable article, it is necessary to replace a developer cartridge detachable from the developer device body or directly supply a new developer to the inside of the developer device after the developer is used up.
When the new developer is supplied, a deteriorated developer remaining in the developer device is mixed with the new developer in the developer device. At this time, an electric charge moves between the new developer and the deteriorated developer, and the new developer is charged in a polarity to be properly charged, and the deteriorated developer is charged in a polarity opposite to the polarity to be properly charged. Consequently, the new developer and the deteriorated developer aggregate electrostatically and an aggregate is generated. When this aggregate or the oppositely charged developer (developer charged in the polarity opposite to the polarity to be properly charged) is supplied from the supply roller to the image carrier through the developing roller, for example, the developer is transferred to the portion other than an image to be formed, and image quality may reduce.
JP-A-8-202143 discloses a developer device including a conductive developing chamber and a supply roller made of a foamable material, and the bottom portion of the conductive developing chamber is arranged to oppose to the supply roller with an interval of, for example, 1 mm. A bias applied to the bottom portion of the developing chamber is a bias that deviates to the same polarity side as a charging polarity of a developer than a bias applied to the supply roller. According to the developer device, an oppositely charged developer adhering to the supply roller is adsorbed to the bottom portion of the developing chamber. As a result, an influence of an aggregate or the oppositely charged developer on image formation can be suppressed.
Incidentally, a supply roller supplies a developer to a developing roller, so that it is particularly desirable to surely remove an oppositely charged developer adhering to the supply roller.
However, the developer device described above provides a relatively large gap (for example, 1 mm) between the supply roller and the bottom portion of the developing chamber, and a developer is introduced in the gap. Therefore, it is difficult to surely adsorb the oppositely charged developer adhering to the supply roller to the bottom portion of the developing chamber. As a result, in the developer device, the oppositely charged developer remaining on the supply roller may be supplied to an image carrier through the developing roller and transferred to a recording sheet. Therefore, a reduction in image quality cannot be suppressed surely.